A bicycle includes a seat tube with a bottom bracket disposed at a bottom portion, a crank axle mounted rotatably on the bottom bracket, a chain wheel mounted on the crank axle, a chain stay disposed to extend rearwardly from the bottom bracket in a longitudinal direction, a seat stay which has an upper end connected to the seat tube and a lower end formed with the chain stay to define a rear bracket, a rear shaft disposed to extend in a transverse axis and which has left and right ends secured relative to the chain stay, a rear wheel hub mounted rotatably on the rear shaft between the left and right ends, a free wheel disposed on the rear wheel hub, and a chain trailed over the chain wheel and the free wheel to transmit rotation of the chain wheel to the rear wheel hub.
A disadvantage of the aforesaid bicycle resides in that whenever the rider wishes to adjust the tension of the chain, the rear shaft must be loosened relative to the chain stay and moved relative to the chain wheel prior to the tightening operation. Two hands must be used in order to hold two ends of the rear shaft during the chain tension adjustment operation in order to avoid misalignment of the rear shaft relative to the transverse axis, thereby inconveniencing the rider.